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BOSTON — The popular pain reliever ibuprofen blocks the heart-protecting effects of aspirin, according to a study that sounds a warning for people who take both medicines.

“It would not do you a lot of good to take one medication only to have another wipe out its effects,” said Dr. Muredach Reilly, a University of Pennsylvania cardiologist who took part in the 30-patient study reported in Thursday’s New England Journal of Medicine.

Many heart patients regularly take aspirin because it thins the blood and prevents the clots that cause heart attacks. Ibuprofen, which is in Motrin and Advil, is widely used for arthritis and other aches and pains.

In the study, when patients took a single dose of ibuprofen beforehand, aspirin lost 98 percent of its blood-thinning power. When aspirin was taken first, three daily doses of ibuprofen sapped aspirin of 90 percent of its benefit.

The researchers believe that ibuprofen clogs a channel inside a clotting enzyme known as cyclooxygenase-1. Aspirin gets stuck at the bottleneck and cannot reach its own active site inside the enzyme.

The study found show no conflict between aspirin and three other arthritis drugs: rofecoxib, diclofenac, and acetaminophen, which is in Tylenol. But the researchers suggested that other drugs with structures like ibuprofen, such as indomethacin, will similarly block aspirin.

Ibuprofen belongs to a widely used class of pain relievers known as nonsteroidal anti-inflammatory drugs.

“This isn’t an indictment of all nonsteroidals, but it does give one pause,” said Dr. Leslie Crofford, an arthritis specialist at the University of Michigan. She wrote an accompanying editorial.

She said researchers should now study humans to verify if these laboratory findings translate into a real danger of heart attacks.

The study was funded partly by the National Institutes of Health and aspirin maker Bayer.

Fran Sullivan, a spokesman for Advil maker Whitehall-Robins Healthcare of Madison, N.J., said if the study is right, “it’s more a matter of timing.” He suggested that regular aspirin be taken two hours before ibuprofen. He said enteric-coated aspirin, which is released more slowly into the blood, could be taken at bedtime without a conflict.

On Thursday, the journal also published a separate study on unintended effects of aspirin and acetaminophen. The study, overseen at the Karolinska Institute in Stockholm, Sweden, showed a 2 1/2 times greater risk of chronic kidney failure in patients who regularly take either drug. Earlier research suggested similar side effects.